Frame relay is a broadband packet switching technology that is often used to implement wide area networks (WANs). Many local and inter-exchange carriers offer frame relay service with access rates ranging from fractional T1 (e.g., n.times.64 Kb/s) to multimegabit (e.g., 44.736 Mb/s T3). Pricing is usually determined by the access line rate, the number of permanent virtual circuits (PVCs) managed by the network and the bandwidth consumed by each PVC. Frame relay is defined by American National Standards Institute (ANSI) specification T1.606, published in 1990 and entitled "Telecommunications--Integrated Services Digital Network (ISDN)--Architectural Framework and Service Description for Frame-Relay Bearer Service" (hereinafter, "the frame relay specification").
FIG. 1 depicts a prior art network configuration 10 in which a frame relay network 12 is used to interconnect three local area networks (LANs) 16a, 16b, 16c. Each of the LANs 16a, 16b, 16c is used to interconnect a respective set of LAN stations 18a, 18b, 18c (e.g., personal computers, workstations or larger computers) and may employ any of a number of different data link layer protocols, including Ethernet, Fiber Distributed Data Interface, Token Ring, and so forth. Each of the LANs 16a, 16b, 16c is coupled to the frame relay network 12 via a respective router 14a, 14b, 14c that typically includes a frame relay packet assembly/disassembly (PAD) function to assemble data received from various LAN stations 18a, 18b, 18c into one or more frame relay packets and to disassemble frame relay packets received from the frame relay network 12 into a format according to the LAN protocol. Although each router 14a, 14b, 14c is depicted as being coupled only between the frame relay network 12 and a respective LAN 16a, 16b, 16c, a router will typically be used interconnect a LAN to several different networks.
To support LAN-to-LAN communications across the frame relay network 12, respective addresses called data link connection identifiers (DLCIs) are usually assigned to each of the LAN stations 18a, 18b, 18c. One DLCI is placed in the address field of each packet carried by the frame relay network to indicate the packet's destination. Because the DLCI effectively steers a packet through the frame relay network 12 to the indicated destination, the DLCI is said to establish a virtual circuit through the frame relay network 12. Permanent virtual circuits (PVCs) are virtual circuits in which the connections between the routers 14a, 14b, 14c and the frame relay network 12 are configured by the provider of the frame relay network 12 and remain established thereafter. Switched virtual circuits, by contrast, require special setup and termination messages to be issued to the frame relay network 12 to establish and terminate a connection.
Still referring to FIG. 1, the connection between a router 14a, 14b, 14c and the frame relay network 12 is a demarcation point referred to as a UserNetwork Interface (UNI), with equipment on the user side of the UNI (e.g., the router, the LAN and the LAN stations) usually being customer premise equipment (CPE) and equipment on the network side of the UNI usually being WAN provider equipment. The router 14a, 14b, 14c is commonly referred to as a frame relay access device (FRAD) because it provides customer premise equipment access to the frame relay network 12.
In 1990, a Consortium of companies including Cisco Systems, Inc., Digital Equipment Corporation, Northern Telecom, Inc. and StrataCom, Inc. developed a link monitoring interface over the UNI called the Local Management Interface (LMI) to allow customer premise equipment to monitor the status of PVCs in a frame relay network. The LMI protocol and its suite of messages are defined by an extension to the frame relay specification published by the Consortium on Sep. 18, 1990 and entitled "Frame Relay Specification with Extensions Based on Proposed T1S1 Standards, Document 001-208966". A later published ANSI standard defines a modified version of LMI ("Integrated Services Digital Network(ISDN)--Signaling Specification for Frame Relay Bearer Service for Digital Subscriber Signaling System Number 1 (DSS1), ANSI T1.617 Annex D", published in 1991). Fundamentally, the Consortium-specified LMI (hereinafter, Consortium LMI) and the ANSI T1.617 Annex D-specified LMI (hereinafter, Annex D LMI) are the same in that a FRAD issues status enquiry messages to the frame relay network 12 and the frame relay network 12 responds with status messages. Because LMI messages can become quite long and assume a one-to-one correspondence between DLCIs and PVCs, existing LMI implementations present obstacles to the transmission of voice and data over frame relay through a single UNI.
One characteristic of frame relay networks is that frame relay packets are permitted to vary in length from one packet to the next. This is in contrast to cell relay networks (e.g., FastPacket networks or Asynchronous Transfer Mode (ATM) networks) in which packets are fixed length cells. One advantage of permitting variable length packets is that, at least in larger packets, the ratio of overhead information (e.g., framing, addressing and error checking information) to payload is relatively small, meaning that a relatively small portion of network bandwidth is consumed by transmission of overhead information. By contrast, relatively short, fixed length cells (e.g., 24 or 53 octets) typically have a larger ratio of overhead to payload so that a larger portion of network bandwidth is consumed by transmission of overhead information. On the other hand, a significant disadvantage of permitting variable length packets to be transmitted on a frame relay network is that variable transmission delays are incurred as packets are queued behind one another in the network's various ingress and egress queues. As a result, data that requires a relatively fixed interval to be maintained between successive packets (e.g., packetized voice, video and other constant bit rate data) becomes distorted by the variable delays in the transmission path. This distortion is called jitter and is one reason that frame relay networks traditionally have not been used to carry voice and other constant bit rate data.
FIG. 2 illustrates a prior art network configuration 21 that allows packetized voice to be transmitted over a frame relay network 12 with significantly reduced jitter. Devices called fragmenters 22a, 22b, 22c receive variable length frame relay packets from respective routers 14a, 14b, 14c and decompose packets that are longer than a predetermined number of octets into two or more smaller packets called fragments. Each fragmenter 22a, 22b, 22c also receives voice inputs and packetizes them into fixed-length packets referred to herein as voice frames. The voice frames and the fragments adhere to the frame relay packet format and are carried by the frame relay network 12 to a destination (e.g., a LAN station 18a, 18b, 18c on a destination network 16a, 16b, 16c) indicated by their respective address fields. Because the voice frames and the fragments are transmitted to the frame relay network 12 on the same access line, long data packets would ordinarily introduce significant jitter to voice frames queued behind them. However, by decomposing long packets into relatively short fragments and then transmitting relatively short fragments across the frame relay network 12, voice frame jitter is significantly reduced. Also, different PVCs can be allocated to carry the fragments and the voice frames through the frame relay network 12 and the PVC used to carry voice frames can often be tailored for voice support. For example, it is usually more important to maintain the relative timing of a sequence of voice frames than to avoid losing frames. Consequently, the PVC for voice may be configured to have a short queue depth and to discard older frames so that if the PVC becomes congested, older voice frames will be discarded instead of being buffered in a deep queue.
FIG. 3 illustrates decomposition of a frame relay packet 24 into fragments 26a, 26b, 26c according to a prior art technique. The frame relay packet 24 includes framing flags at its beginning and end (FLAG), two octets of addressing information (ADDR) and a two octet frame check sequence (FCS). The frame check sequence is typically cyclic redundancy check value (CRC). The packet 24 also includes a variable length information field (i.e., a payload) that includes N octets of data. As indicated in FIG. 3, respective portions of the original packet 24, not including the flag octets, are copied into payload sections of successive fragments 26a, 26b, 26c. The payload section of each fragment is limited to K octets so that approximately (N/K)+1 fragments are required to represent the original packet 24. In order to delineate one sequence of fragments from the next and also to ensure that the fragments are properly applied to restore the original packet 24, a last flag and a sequence number is included in the address field (ADDR) of each fragment 26a, 26b, 26c. The sequence number is incremented for each successive fragment 26a, 26b, 26c to supply fragment ordering information, and the last flag set to FALSE for each fragment 26a, 26b, 26c except the last. When a fragment having a TRUE last flag is received at a remote fragmenter, the sequence number of the fragment indicates to the remote fragmenter the total number of fragments required to reconstruct the original packet 24.
Using the fragmenting techniques described above, it is possible to transmit voice frames and fragmented data packets over the same UNI without causing unacceptable jitter in the voice frame delivery. At least one problem that remains, however, is that LMI status messages are not fragmented by a fragmenter and can become long enough to noticeably interfere with voice transmission. More specifically, LMI status messages typically include at least five octets for each allocated PVC and therefore will exceed a maximum packet length if the number of allocated PVCs rises above a predetermined number. If the LMI status message substantially exceeds the maximum packet length, a periodic "glitch" may be heard on the voice output as each full LMI status message is transmitted. This is undesirable, of course, and can be avoided by limiting the number of allocated PVCs. However, because the number of PVCs in a frame relay network is usually determined by the number of assigned DLCIs, and because an additional DLCI is typically assigned for each new LAN station that is connected for access to the frame relay network, it is often difficult to limit the number of allocated PVCs and yet keep up with demand for additional LAN station connections.
As mentioned above, a key factor for pricing subscriber access to a frame relay network is the number of PVCs allocated to the subscriber. One reason for this is that congestion management, LMI and other network management functions are performed on a per-PVC basis. Another reason is that the supply of PVCs available on a frame relay network is limited by the available number of DLCIs. In a frame relay network, one PVC is allocated for each ten bit DLCI assigned to subscriber equipment. The ten bit format allows up to 1024 DLCIs to be assigned per frame relay network. However, because one DLCI is typically assigned to each LAN station that has access to the frame relay network, DLCIs are quickly consumed as LAN station connections increase. Thus, it can be seen that allocating a large number of PVCs in a frame relay network presents a manifold problem: frame relay network resources are strained, the cost to network subscribers is relatively high and LMI status messages become so long as to interfere with transmission of voice frames through the network
One technique for reducing the number of PVCs required in a frame relay network is to bundle multiple voice PVCs together under a single DLCI. This is accomplished by including multiplexing information in each of the voice packets transmitted to the frame relay network. From the perspective of the frame relay network, the bundled voice PVCs appear to be a single PVC because only one DLCI is allocated. However, when voice packets are received in a remote fragmenter or other frame relay access device that understands the sub-multiplexed addressing format, the remote device can use the multiplexing information to distinguish one PVC from another in a bundle. The voice packets can then be distributed to telephony equipment connected to the remote device according to the multiplexing information. The overall effect is to provide multiple PVCs in a bundle that appears to a frame relay network to be a single PVC.
FIG. 4 is a diagram of a prior art voice frame 30 that includes multiplexing information to allow a PVC to coexist with other PVCs in a bundle identified by a single DLCI. PVCs that are bundled under a shared DLCI are referred to herein as "sub-multiplexed PVCs to distinguish them from the "bundling PVC" perceived by the frame relay network. Each voice frame 30 transmitted on a sub-multiplexed PVC adheres to the frame relay packet structure and includes a flag octet (0111 1110=7E hex) followed by a standard, two-octet frame relay address field. The first octet of the address field includes the most significant six bits of the DLCI, followed by a command/response bit (C/R) and an extended address bit (EA). The second octet of the address field includes the least significant four bits of the DLCI followed by the forward and backward explicit congestion notification bits (FECN and BECN), a discard eligibility bit (DE) and another extended address bit. The explicit congestion notification bits are used to indicate the direction of network congestion, if any, and the discard eligibility bit is used to determine whether the frame 30 may be discarded by the network. The extended address bit is present in each octet of the address field and is zero for each octet in the address field except the last. This provides a mechanism for including additional octets in the address field if necessary. The next octet after the address field provides the multiplexing information described above and is referred to as a multiplexing value. In effect, the multiplexing value extends the precision of the destination address to allow sub-multiplexing of PVCs under a common DLCI. Each sub-multiplexed PVC is identified by a different multiplexing value. The multiplexing value is followed by the digitized voice information and then by a FCS. A flag octet ends the voice frame 30.
Although PVCs used to carry voice frames can be sub-multiplexed under a shared DLCI, PVCs used to carry bursty data packets are typically not sub-multiplexed. One reason for this is that the Consortium and Annex D LMI protocols do not provide the granularity of link status information needed to support PVC bundling. As discussed above, a one-to-one correspondence between PVCs and DLCIs typically exists in a frame relay network and link status information is provided on a per-DLCI basis. Consequently, if PVCs used to carry bursty data were bundled under a single DLCI, the LMI status response message returned by the frame relay network would indicate only the link status of the bundling PVC, not the link status of the individual sub-multiplexed PVCs. This presents a serious impediment to bundling bursty data under a shared DLCI because a connection failure in one sub-multiplexed PVC may be incorrectly reported by the frame relay network to be the failure of each sub-multiplexed PVC in the bundle. When failure of the bundle is reported to the router, multiple connections may unnecessarily be downed. At least partly for this reason, PVCs used to carry bursty data are usually not sub-multiplexed and a large number of PVCs may be required in the frame relay network to support connection of LAN stations and other devices that transmit and receive bursty data. Consequently, the above described problems of network resource depletion, LMI status interference with voice frames and relatively costly frame relay network access remain.